1. Field of the Invention
The present invention generally relates to a noise removing apparatus in an FM receiver. More particularly, the present invention relates to an improved noise removing apparatus for removing a multipath noise utilizing a prior pulsive noise removing circuit.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an example of a prior art noise removing apparatus in an FM receiver where the present invention can be advantageously employed. The example as shown in FIG. 1 shows an FM stereo receiver. The FM stereo receiver comprises an antenna 1 for receiving a broadcast FM signal wave, a high frequency amplifier 3 for amplifying the FM signal received by the antenna 1, a local oscillator 7 for generating a local oscillation signal for the purpose of frequency conversion, a mixer 5 for mixing the amplified FM signal from the high frequency amplifier 3 with the local oscillation signal for converting the frequency of the FM signal into an intermediate frequency, an intermediate frequency amplifier 9 for amplifying the intermediate frequency signal from the mixer 5 and an FM detector 11 for demodulating the intermediate frequency signal into the original low frequency signal. The low frequency signal from the FM detector 11 is applied to a stereo multiplex circuit 19 through a delay circuit 13, a gate 15 and a store/pilot signal generating circuit 17. The stereo multiplex circuit 19 demodulates the provided low frequency signal into the original stereo signal. The left and right low frequency signals from the stereo multiplex circuit 19 are applied to left and right loud speakers 25 and 27, respectively, through a left low frequency amplifier 21 and a right low frequency amplifier 23. Detailed structure and operation of the respective blocks are well known to those skilled in the art. Hence, it is not believed necessary to describe the same here in more detail.
Circuits 13, 15 and 17 are interposed between the FM detector 11 and the stereo multiplex circuit 19 for removing a noise. The delay circuit 13 delays, say for 3 to 5 microseconds the low frequency signal from the FM detector 11 to provide the delayed signal to the gate 15. The gate 15 is interrupted when a pulsive noise is detected by a pulsive noise detecting circuit 31 as described subsequently and prevents the pulsive noise from being transmitted to the subsequent stage. The store/pilot signal generating circuit 17 is of a circuit for preventing a stereo pilot signal from being disturbed by opening or closing the gate 15. Such store/pilot signal generating circuit is typically seen in U.S. Pat. No. 4,066,845, issued Jan. 3, 1978 to the same assignee as the present invention and U.S. Pat. No. 3,739,285, issued June 12, 1973.
The low frequency signal from the FM detector 11 is also applied to a high-pass filter 29. The high-pass filter 29 is designed to detect the energy of a noise component included in the output of the FM detector 11 and is adapted to pass the signal component of the frequency (however, the frequency must be higher than an SCA band, in case of an SCA broadcast) higher than the stereo composite signal. The pulsive noise detector circuit 31 is aimed to detect a pulsive noise in the output of the high-pass filter 29 and is adapted to trigger a monostable multivibrator 33 upon detection of such pulsive noise. The monostable multivibrator 33 continues to provide an output for a predetermined time period after the same is triggered, so that the gate 15 is interrupted when the output is obtained from the monostable multivibrator 33. Thus, the pulsive noise can be prevented from being sounded from the loud speakers 25 and 27.
Such pulsive noise removing apparatus is known in U.S. Pat. No. 4,278,901, issued July 14, 1981, U.S. Pat. No. 4,289,981, issued Sept. 15, 1981 and the like and thus it is not believed necessary to describe the same here in more detail. The above described U.S. Pat. Nos. 4,278,901 and 4,289,981 both are assigned to the same assignee as the present invention.
A prior art noise removing apparatus as shown in FIG. 1 improves various points and, particularly, brings about an excellent meritorious effect as far as the removal of a pulsive noise is concerned. However, no multipath noise can be considered in any of prior art apparatuses. The multipath noise means a noise caused by a phase difference between received signals including a reflective signal wave as well as a direct signal wave which are received by a receiver. In particular in an FM stereo receiver, a stereo pilot signal having a frequency of 19 kHz mainly generates a large noise component, as shown in FIG. 2, due to an adverse effect of the multipath. The noise caused from the pilot signal basically comprises a repetition of 19 kHz and generates an audible noise from a loud speaker when the noise is demodulated. Such multipath noise is frequently generated in case where a receiving point where an FM signal is received by an FM receiver is continuously changed, for example, in case where the FM receiver is loaded in a car, and thus such multipath noise must be fully considered in a so-called car stereo.